it is one of the reasons it took them so long, in order to have social housing
Are you sure it is that way round?
Usually builders only get planning permission to build if they commit to x number of units being affordable housing. And more often than not after building works are complete the LA agrees to reduce the number because of increaesed costs, blah, blah.
Also, "affordable" housing means rents at something like 80% of commercial rents, which (particularly London) means unaffordable rents.
Affordable rents are not the same as social housing rents.